Many rare earth-transition metal alloys are known in the art to form high energy permanent magnet materials. Samarium cobalt magnets have received much attention, however, because of economic considerations the trend has been toward other more plentiful and therefore cheaper materials. Alloys of neodymium and or praseodymium are particularly suitable from both the properties standpoint and from the economic standpoint. Particularly suitable alloys of this class are those where the particular rare earth is combined with iron and boron. European Patent Application 0 108 474 published May 16, 1984 teaches a method of making isotropic magnets by hot pressing of melt spun ribbons. European Patent Application No. 0 133 758 published July 11, 1984 has as one of its objects to provide a fully densified fine grain, anisotropic, permanent magnet formed by hot working a suitable material comprising iron, neodymium and or praseodymium and boron.
While these magnets show some degree of anisotropy, as evidenced by the second quadrant demagnetization curve wherein remanence in the preferred direction is compared with the remanence far removed from the preferred direction, it is significantly less than two in all examples shown. In addition, the technique employed to obtain the degree of anisotropy obtained is expensive and requires machining of the magnets for applications such as use in rotating machines including stepping motors, multi-pole rotors; beam focusing devices, magnetic electrographic development rollers and the like where the magnets preferably should possess anisotropic properties in the radial direction.
In copending U.S. application Ser. No. 159,160 filed on even date herewith entitled "Anisotropic High Energy Magnets and a Process of Preparing the Same" by D. K. Chatterjee and assigned to the same assignee as this application, is described a method of making anisotropic permanent magnets of a rare earth magnetic alloy by extruding the alloy at a temperature below the melting point thereof and at an extrusion ratio of from above 10 to 1 to about 26 to 1. By control of the extrusion temperature, the extrusion ratio and the shape of the extrusion orifice, the preferred alignment of the fully dense magnets can be predetermined and controlled. For example, should it be desired to produce a cylinder or a hollow roller having anisotropic properties in the radial direction the rare earth alloy can be extruded through a circular orifice or a mandreled or annular ring orifice to obtain this preferred alignment.
Because of the corrosive nature of these materials, especially as the particle size of the particles employed are reduced, a problem is encountered that being corrosion of the rare earth magnetic alloys both during processing and thereafter. Also, difficulty is encountered in obtaining fully dense magnets.